1. Field of the Invention
The present invention relates to a process for producing a glass member (preform) used for making glass fibers for optical transmission.
2. Description of the Prior Art
The properties required of a glass preform for a glass fiber are it should have a certain definite refractive index distribution in the radial direction (when the preform is in the shape of a rod); the concentration of the dopant in the preform should be substantially free from variation; the concentrations of impurities such as transition metals (e.g., iron or copper) or hydroxyl groups should be low; and the preform should transmit light well.
One method for producing such a preform at low cost is disclosed in Japanese patent application (OPI) Nos. 28339/75, 126207/76 and 135915/76 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and involves doping a porous glass body made of a phase-separable glass with a dopant capable of changing the refractive index of the glass.
Such a method, however, has the following disadvantages.
(i) In order to reduce the cost of production, powdery materials (such as SiO.sub.2, K.sub.2 CO.sub.3, Na.sub.2 CO.sub.3 and H.sub.3 BO.sub.3) with large amounts of impurities are used as starting materials, and removal of the impurities is attempted by utilizing the fact that the impurities are concentrated into the soluble phase during the step of phase separation by heat-treatment. The effect of this, however, decreases as the concentration of the impurities in the starting materials decreases.
(ii) Use of high-purity starting materials in an attempt to reduce the amount of impurities, on the other hand, offsets the advantage of reduced cost.
(iii) The pores of the porous glass body must be uniformly distributed throughout the entire glass body to dope the glass with a given dopant and achieve uniform vitrification. A phase-separable glass, however, is thermal history dependent, and the pores of such a glass are not uniform.
(iv) Moisture present in the starting materials and introduced at the time of melting the glass results in hydroxyl groups remaining in a B.sub.2 O.sub.3 --SiO.sub.2 glass which forms a matrix of the porous glass body. Removal of the hydroxyl groups is by no means easy even if various modified heat-treatment procedures are utilized.
Japanese patent application (OPI) Nos. 119878/74 and 73908/75 disclose a process for preparing a transparent glass body which comprises immersing a fine silica powder obtained, for example, using a flame hydrolysis method in an aqueous solution to impregnate the pores with salts and sintering the silica powder. Such a method, however, has the following disadvantages.
(i) If flame hydrolysis is to be used to produce somewhat solid masses of fine silica glass powder which do not break in an aqueous solution, the powder at the center of a mass is exposed to heat for longer periods of time as the size of the powder mass increases. Hence, sintering advances further at the center of the mass, and a difference in porosity arises between the outside of the powder mass and the powder mass near its center. Consequently, it is difficult to control the porosity of the powder mass.
(ii) If the fine glass powder mass is to be hardened to some extent, the temperature for reacting the fine glass powder as well as the temperature of the fine glass powder produced must be increased. As a result, the temperature of the fine glass powder mass that serves as a target onto which powder is subsequently deposited becomes high, and the rate of adhesion of the fine glass powder is decreased.
(iii) If a fine glass powder mass having a controlled porosity is to be produced in a good yield, the resulting glass powder mass becomes so soft that it easily breaks in an aqueous solution. This poses a serious problem when controlled doping is to be carried out by changing the temperature or the concentration of the aqueous solution.